Recently, the requirements for improvement of fuel economy to land-travelling vehicles on which a gasoline engine or a diesel engine is mounted, such as automotives, have continued with increased severity. It has been known that the improvement of fuel economy is most effectively made by reduction of weight of the vehicle on which an internal combustion engine is mounted. It also has been known that the improvement of fuel economy can be attained by appropriately selecting the lubricating oil composition employed for lubricating the internal combustion engine mounted on the vehicle.
For the reason described above, until now, the companies engaged for developing lubricating oil compositions as well as the companies engaged for developing lubricating oil additives have intensively made studies for providing lubricating oil compositions which show improved fuel economy.
The improvement of fuel economy provided by the lubricating oil composition generally can be attained by reduction of torque of rotation of the engine parts which is caused by the use of a lubricating oil composition providing reduction of friction to the sliding parts of the engine. The extent of reduction of friction provided by the lubricating oil composition mainly depends on the viscosity of the lubricating oil composition and selection of additives, mainly the selection of the friction modifier. For instance, if the lubricating oil composition is formulated with a base oil of low viscosity, the friction-reducing effect provided by the lubricating oil composition increases. However, the use of the base oil of low viscosity likely results in unfavorable phenomenon such as increase of oil consumption occurring during operation of the engine. Therefore, the improvement of fuel economy by the use of a lubricating oil composition employing a base oil of low viscosity has its limit. For this reason, intensive studies have been made on employment of appropriate lubricating oil additives, whereby improving the fuel economy provided by the lubricating oil composition. Until now, a variety of inventions made from these studies have been disclosed in a number of patent applications.
Patent publication 1 (JP 5-163497 A) discloses an engine oil composition showing low friction property, which comprises a base oil and an additive composition comprising a borated alkenylsuccinimide, an alkaline earth metal salicylate and molybdenum dithiophosphate and/or molybdenum dithiocarbamate.
Patent publication 2 (JP 6-336592 A) describes that a lubricating oil composition comprising zinc dialkyldithiophosphate being mainly composed of a zinc dialkyldithiophosphate having a secondary alkyl group, a metal-containing detergent composition comprising calcium sulfonate (e.g., TBN 200-300) and calcium salicylate (e.g., TBN 10-100), and molybdenum dithiocarbamate having a C8-23 hydrocarbyl group shows increased low wear-producing property and increased low friction property.
Patent publication 3 (JP 8-302378 A) describes that an engine oil composition comprising a base oil having a specific kinematic viscosity and specific total aromatic content, an alkaline earth metal salicylate, zinc dialkyldithiophosphate, a succinimide ashless dispersant having a polybutenyl group having a molecular weight of 900-3500, a phenol type-ashless oxidation inhibitor, and molybdenum dithiocarbamate shows improved fuel economy.